I have a rather old Kurzweil PC88mx. I have generally used it for playing but only occasionally used the MIDI input and output. The internal battery is now extremely low, and I have not bothered to replace it, since the general operation of the keyboard is fine with it plugged in. However, I now cannot get the MIDI input or output to work. Does MIDI on the PC88mx require a viable internal battery, or should it be OK when the keyboard is plugged in?